Upon completing Underhalls, you get the dialouge that you would usually hear at the end of the Highblock Halls level ("The Arch-Illager escaped to the ramparts. Press on Hero, you mustn't let him get away!").

See attached video file to see the end of the underhalls level. (lack of audio was just my recording messing up)

Unsure how to add the "1.8.6.0" tag to the already existing bug report's Affects Versions field, so...